Heater box...maybe I left a piece out?

Bought a '72 Scamp a couple of years ago. 53,000 mile car. First fall I had it, you would have thought I had the vent doors open. Needed to replace the heater core the next spring and while removing the heater box, I noticed a large gap between the box and the cowl. The box didn't have this rod in it and there was no hole in the bottom of the heater box either. It had the "slots" molded into it, but no hole and no rod. Purchased a rod from a fellow FABO member and now no breezes.